Flat monolithic electromechanical systems, for example accelerometers micro-machined from a silicon wafer, conventionally comprise a body having a base and a measurement cell and often two measurement cells in order to implement a differential method. A measurement cell typically comprises a proof mass linked on the one hand to the base and on the other hand to a force sensor, itself also linked to the base. When the system is subjected to an acceleration along the sensitive axis which is the direction of the acceleration to be measured, the proof mass is subjected to an inertia force which is transmitted to the force sensor by means possibly making it possible to amplify the transmitted force.
The patent application FR 02 15599 describes an accelerometer whose measurement means comprise a resonator which can be a vibrating beam.
Hereinafter, the resonator will be exemplified by two beams forming a tuning fork, made to vibrate in phase opposition by means of two electrodes. It is this tuning-fork configuration which is represented in FIG. 1. Other resonator structures or a balance of forces could equally be used as measurement means. A vibrating-beam accelerometer preferably has two measurement cells which can be produced by micro-machining a substrate of silicon on insulator (SOI) or of quartz or another material, but other methods are also possible.
A silicon on insulator substrate comprises a wafer of silicon, possibly monocrystalline, measuring a few hundreds of micrometers thick (450 micrometers for example) forming the base of the accelerometer, which bears on its front face a thin layer of silicon oxide a few micrometers thick (2 micrometers for example) which is in turn covered by a layer of silicon a few tens of micrometers thick (60 micrometers for example), the silicon possibly also being monocrystalline.
The machining consists in etching the layers of silicon through their external face toward the layer of oxide, with a selective etching method which etches the silicon without significantly etching the oxide. The etching can be interrupted when the layer of oxide is exposed. This layer of oxide can in turn be removed locally by selective etching by another method so as to retain a link between the two layers of silicon only at a few selected points, and thus obtain the desired mobile planar structure.
Hereinafter a frame of reference O,x,y,z will be used. The plane of FIG. 1 is the plane O,x,y, the axis Oz representing the direction perpendicular to this plane. The axis Ox (respectively Oy, Oz) designates an axis parallel to the axis Ox (respectively Oy, Oz) represented in the figure.
The mobile planar structure 10 of a measurement cell of the accelerometer, diagrammatically represented in FIG. 1, comprises a mobile proof mass 1 capable of translation displacement along the sensitive axis of the accelerometer designated axis Oy, which is parallel to the acceleration γ to be measured, and means 2 of amplifying the force limiting this translation. This force is measured by means of two vibrating beams 30 placed along an axis Ox perpendicular to the axis Oy, which undergo a traction or a compression in the direction of the acceleration. The beams are arranged symmetrically relative to an axis of symmetry S of the structure, this axis of symmetry being parallel to the axis Oy and passing through the center of gravity of the mass.
The vibrating beams 30 are embedded at each end in a rigid termination 4. Each of these terminations 4 has a pair of micro-machined arms. The two pairs are symmetrical relative to the axis of symmetry S. A first micro-machined arm 5 links the termination 4 to the proof mass 1. A second micro-machined arm 6, symmetrical to the first arm relative to the axis of the beam, links the termination 4 to an anchoring foot 7 fixed to the base. These arms 5 and 6 are respectively linked to the proof mass, to the termination 4 and to the anchoring foot 7, by attachment points. The thickness of an arm 5 or 6 can vary over its length.
Also diagrammatically represented in FIG. 1 is a detail of part of the amplification means. The first arm 5 is articulated on the termination 4 by an attachment point A. Also represented is a part of electrode E. The two vibrating beams 30 are embedded in the termination 4 insofar as they are formed by etching for example, of the same layer of material. The section lines represent the material, monocrystalline silicon for example in the case of a cell produced by machining an SOI. As indicated previously, the surface patterns such as the arms 5, attachment point A, termination 4, beams 30 and electrode E, have been obtained by engraving the monocrystalline silicon then by etching the layer of oxide.
The angle α formed by the axis Ox and the line joining the attachment points A and B of the first arm 5 which, because of the symmetry of the arms 5 and 6 relative to the axis linking the terminations through their middle, is symmetrical with the angle formed by the axis Ox and the line joining the attachment points of the second arm 6. This angle α is relatively small and the traction or compression force exerted on the beams 30 is greater than the inertia force generated by the proof mass 1 in the presence of a sensitive acceleration directed along the axis Oy.
These amplification means 2 also make it possible to free space around the vibrating beams 30, notably to place the electrodes in the case of an electrostatic excitation. It will be recalled that the vibrating means are vibrated at their resonance frequency using electrodes arranged facing these beams, or directly on the beams, depending on whether it is an electrostatic or piezoelectric excitation.
The proof mass 1 is partially guided in translation along the axis Oy by the symmetry of the structure. To retain a degree of translation freedom solely along the axis Oy, it is also possible to limit the translations of the proof mass along Ox and Oz as well as rotations by employing one or more guiding blades 8 oriented along the axis Ox. A first end of this blade 8 is fixed to the proof mass 1, a second end of this blade 8 is fixed to a part 9, fixed to the base. Conventionally, the guiding blades 8 have a rectangular parallelepidedal form, with the longest side placed along the axis Ox.
FIG. 2 represents, in plan view, that is to say according to a projection in the plane O,x,y, a guiding blade 8 according to the prior art of a flat electromechanical system micro-machined from a wafer of monocrystalline silicon of constant thickness h. The guiding blade 8 comprises two anchorages and a section: the first anchorage is incorporated in the proof mass 1, a second anchorage is incorporated in the base.
As stated above, the section of a guiding blade of the prior art has a rectangular parallelepipedal form. The parallelepiped formed by the section of the blade 8 has a thickness along the axis Oz equal to the thickness of the substrate from which the electromechanical system is machined; for example h is equal to 60 micrometers.
The parallelepiped also has a length, along the axis Ox, equal to I1, for example I1 of the order of 700 micrometers, and a width along the axis Oy equal to L, for example L is close to 5 micrometers.
It is essential for the width L to be very small in order to present a very low resistance to a movement of the proof mass 1 in the direction Oy. The width L is generally given a value which is the minimum that can be reached with the available substrate machining technologies.
The dimensions of the guiding blade 8 give it a stiffness coefficient value along the axis Oy, ky, that is extremely low, typically two orders of magnitude less than the stiffness along the axis Oy of the inertia force measuring device. On the other side of the coin, a guiding beam which has such a geometry has a critical buckling load that is very low along the axis Ox, typically corresponding to an acceleration of the order of 1000 g along the axis Ox, that is to say equal to a thousand times the acceleration of gravity on the Earth's surface. This therefore prohibits the use of such a guiding blade in a micro-machined electromechanical system which would be subjected to accelerations of the order of 20000 g along the axis Ox because, in this case, a buckling phenomenon would arise, and the guiding blade would be very greatly deformed and more than likely damaged.
To withstand such an acceleration along the axis Ox, it would be necessary for the width L of the blade along Oy to have a high value, for example greater than 50 micrometers for a length along the axis Ox that is unchanged in order to present a high critical buckling load along the axis Ox, which is incompatible with a low stiffness along the axis Oy.
The guiding blade according to the prior art, for example when it has a substantially parallelepipedal form, has a form that makes it unsuited to operation under a very high value acceleration along the axis Ox.
The aim of the invention is to overcome this drawback.